It is a common practice to treat subterranean formations to increase the gross permeability or conductivity of such formations by procedures which are identified generally as fracturing processes. For example, it is a conventional practice to hydraulically fracture a well in order to produce one or more cracks or "fractures" in the surrounding formation by mechanical breakdown of the formation. Fracturing may be carried out in wells which are completed in subterranean formations for virtually any purpose. The usual candidates for fracturing, or other stimulation procedures, are production wells completed in oil and/or gas containing formations. However, injection wells used in secondary or tertiary recovery operations, for example, for the injection of water or gas, may also be fractured in order to facilitate the injection of fluids into such subterranean formations.
Hydraulic fracturing is accomplished by injecting a hydraulic fracturing fluid into the well and imposing sufficient pressure on the fracture fluid to cause formation breakdown with the attendant production of one or more fractures. The fracture or fractures may be horizontal or vertical, with the later usually predominating, and with the tendency toward vertical fracture orientation increasing with the depth of the formation being fractured. Usually a gel, an emulsion or a foam, having a proppant, such as sand or other particulate materials suspended therein, is introduced into the fracture. The proppant is deposited in the fracture and functions to hold the fracture open after the pressure is released and fracturing fluid is withdrawn back into the well. The fracturing fluid has a sufficiently high viscosity to penetrate into the formation to realize fracturing and to retain the proppant in suspension or at least to reduce the tendency of the proppant of settling out of the fracturing fluid. Generally, a gelation agent and/or an emulsifier is used to gel or emulsify the fracturing fluid to provide the high viscosity needed to realize the maximum benefits from the fracturing process.
After the high viscosity fracturing fluid has been pumped into the formation and the fracturing of the formation has been completed, it is, of course, desirable to remove the fluid from the formation to allow hydrocarbon production through the new fractures. Generally, the removal of the highly viscous fracturing fluid is realized by "breaking" the gel or emulsion or, in other words, by converting the fracturing fluid into a low viscosity fluid. Breaking a gelled emulsified fracturing fluid has commonly been obtained by adding "breaker", that is, a viscosity-reducing agent, to the subterranean formation at the desired time. This technique, however, can be unreliable, sometimes resulting in incomplete breaking of the fluid and/or premature breaking of the fluid before the process is complete. Premature breaking can decrease the number of fractures obtained and thus, the amount of hydrocarbon recovery. Further, it is known in the art that most fracturing fluids will break if given enough time and sufficient temperature and pressure. It is, of course, most desirable to return the well back to hydrocarbon production as quickly as possible.
There have been several proposed methods for the breaking of fracturing fluids which are aimed at eliminating the above problems. For example, U.S. Pat. No. 5,164,099 discusses several of the proposed methods, the description of which is hereby incorporated by reference. U.S. Pat. No. 5,164,099 discloses inter alia, see claim 1, a method for breaking an aqueous fracturing fluid comprised of introducing an encapsulated percarbonate, perchlorate, or persulfate breaker into a subterranean formation being treated with the fracturing fluid, said encapsulated breaker comprised of a polyamide membrane enclosing the breaker, said membrane permeable to a fluid in the subterranean formation such that the breaker diffuses through the membrane to break the fracturing fluid with said membranes staying intact throughout the method for breaking the fracturing fluid.
The above-identified methods of breaking fracturing fluids use various chemical agents such as oxidants, i.e., perchlorates, percarbonates and persulfates. Oxidants not only degrade the polymers of interest but also oxidize tubulars, equipment, etc. that they come into contact with, including the formation itself. In addition, oxidants also interact with resin coated proppants and, at higher temperatures, they interact with gel stabilizers used to stabilize the fracturing fluids which tend to be antioxidants. Also, the use of oxidants as breakers is disadvantageous from the point of view that the oxidants are not selective in degrading a particular polymer. In addition, chemical breakers are consumed stoichiometrically resulting in inconsistent gel breaking and some residual viscosity which causes formation damage.
The use of an enzyme to break fracturing fluids would eliminate some of the problems relating to the use of oxidants. For example, enzyme breakers are very selective in degrading the specific polymers. The enzymes do not effect the tubulars, equipment, etc. that they come in contact with and/or damage the formation itself. The enzymes also do not interact with the resin coated proppants commonly used in fracturing systems. Enzymes react catalytically such that one molecule of enzyme may hydrolyze up to one hundred thousand (100,000) polymer chain bonds resulting in a cleaner more consistent break and very low residual viscosity. Consequently, formation damage is greatly decreased. Also, unlike oxidants, enzymes do not interact with gel stabilizers used to stabilize the fracturing fluids.
Heretofore known enzyme breakers were of limited use because of their known activity range and sensitivity to metal ions. For example, according to the Kirk-Othmer Encyclopedia of Chemical Technology it is "well known that the folded structure by which catalytic activity [of enzymes] is achieved is destroyed &gt;50.degree.-70.degree. C. (122.degree. F.-158.degree. F.)." Also, U.S. Pat. No. 5,201,370 discloses an enzyme breaker for galactomannan-based fracturing fluids wherein the enzyme breaker is effective to degrade the polymer gel at temperatures between 50.degree. F. and 180.degree. F. (See column 3, lines 55-58 and column 5, lines 42-44) More recently, a hemicellulase enzyme was disclosed in WO 91/18974, which is hereby incorporated herein by reference, having a range of activity from 0.degree. C. (32.degree. F.) to 90.degree. C. (194.degree. F.). Thus, heretofore known enzymes have had activity ranges from 30.degree. C. (86.degree. F.) to 90 .degree. C. (194.degree. F.), i.e., the enzymes were active across the entire range, or a major portion of it, and were not active at all over 90.degree. C. Therefore, heretofore it was believed that enzyme breakers effective over 194.degree. F. were not possible and, consequently, the use of enzyme breakers in many fracturing systems was limited or even impossible. Moreover, heretofore all known enzymes for use as breakers displayed activity across a wide temperature range including low temperatures, thus making their use of limited value.
Also, according to the Kirk-Othmer Encyclopedia of Chemical Technology, "many enzymes are sensitive to transition metal ions that act as inhibitors of enzyme activity." Typically, in fracturing systems, in addition to borates, transition metal chelates such as titanates or zirconates are used as crosslinking agents for guar, cellulose and their derivatives. Thus, heretofore it was also believed that some enzymes would lose activity in the presence of these crosslinkers.
Typically, as set forth above, all enzymes are active at low temperatures between 0.degree. and 70.degree. C. Heretofore, if such an enzyme was used in a fracturing treatment incorporating a polymer gel, the enzyme would very rapidly break the gel and lower the viscosity of the fluid as soon as it came into contact with the gel. The gel would then be ineffective at carrying proppant and the fracturing treatment would fail. An enzyme breaker was needed which is inactive at low temperatures but becomes active at the temperatures encountered in the formation. Such an enzyme breaker could allow the high viscosity gel to carry proppant into the formation where upon it is heated by the formation and begins to break the gel. After a sufficient time, the heated enzyme will effectively catalytically break the polymer gel resulting in low polymer residue in the formation and minimal formation damage.
Therefore, there exists a need for an enzyme breaker having only a limited temperature range activity and being active over temperatures of about 200.degree. F. The invention of the present application provides an enzyme breaker, having the advantages over oxidant breakers, whose activity is effective only over certain specific temperatures selected for a particular application, including applications involving temperatures over 200.degree. F. and is not sensitive to transition metal ions.